Apical–basal polarity in epithelial cells is essential for tissue morphology and function. The polarised formation of cell junctions, such as adherens junctions (AJs), and the dynamic turnover of adhesion complexes are required for the generation and maintenance of cell polarity. But how do AJ dynamics change during the establishment of polarity? Here, Yang Hong and co-workers (p. 4001) use fluorescence recovery after photobleaching (FRAP) assays in Drosophila embryonic epithelial cells to assess the turnover of the two main components of AJs – Drosophila E-cadherin (DE-Cad) and β-catenin (Armadillo, Arm) – in polarising and polarised cells. They find that the biosynthetic delivery of DE-Cad and Arm is much faster in early-stage embryos – where polarity is being established – than in established epithelia. By analysing the diffusion of membrane pools of these proteins into AJs, they also show that the membrane redistribution dynamics of DE-Cad are not affected by AJ maturation. By contrast, Arm diffusion into AJs is slower in mature epithelial cells. This suggests that Arm binds dynamically to DE-Cad during polarisation, and that these proteins only become tightly associated in polarised cells to form mature AJs. Modulating the molecular interaction between these two proteins might, therefore, be crucial for the correct establishment of apical–basal polarity during development.